A helmet protects a skull of the wearer from collisions with the ground, equipment, and other players. Present helmets were designed with the primary goal of preventing traumatic skull fractures and other blunt trauma. In general, a helmet includes a hard, rounded shell and cushioning inside the shell. When another object collides with the helmet, the rounded shape deflects at least some of the force tangentially while the hard shell distributes the normal force over a wider area of the head. Such helmets have been successful at preventing skull fractures but leave the wearer vulnerable to concussions.
A concussion occurs when the skull changes velocity rapidly relative to the enclosed brain and cerebrospinal fluid. The resulting collision between the brain and the skull results in a brain injury with neurological symptoms such as memory loss. Although the cerebrospinal fluid cushions the brain from small forces, the fluid does not absorb all the energy from collisions that arise in sports such as football, hockey, skiing, and biking. Helmets include cushioning to dissipate some of the energy absorbed by the hard shell, but the cushioning is insufficient to prevent concussions from violent collisions or from the cumulative effects of many lower velocity collisions.